TOKYO -- Federal investigators had just bought a 2007 Lexus ES 350 and were running it through a battery of tests at the government's Vehicle Research and Test Center in East Liberty, Ohio, when they noticed something funny with the electronic throttle control.

The vehicle was rigged with monitors to root out possible causes of unwanted acceleration. The engineers, from the National Highway Traffic Safety Administration, were testing yaw rate, speed, acceleration, brake pressure, brake pad temperature, throttle-plate position, gas-pedal position and the brake booster vacuum, among other things.

And when they introduced magnetic fields in proximity to electronic controls for the throttle body and accelerator pedal, the engine mysteriously started revving by itself.

The engine clicked up by only 1,000 rpm -- similar to a cold-engine idle level. Unimpressed, investigators turned their focus to floor mats that could jam the gas pedal and trigger unintended acceleration.

By September 2007, just a month later, Toyota was recalling 2007-08 Lexus ES 350s and Camrys in the United States to fix all-weather mats.

Investigations such as this, detailed in an April 30, 2008, final report issued by NHTSA, are now under fresh scrutiny. What emerges is a picture of lax federal oversight into Toyota's acceleration problem as complaints increased over the past decade.

Critics say that long before Toyota's massive recalls, lawsuits, and allegations of cover-ups, U.S. investigators may have failed to nip defects in the bud. At the center of the storm are enduring doubts about Toyota's electronic control technology.

The Center for Auto Safety, a Washington consumer advocacy group founded by Ralph Nader, is among those charging that NHTSA's investigation of the electronic controls was cursory.

TURMOIL AT TOYOTA

Since last fall, Toyota has recalled Toyota and Lexus vehicles on five continents to resolve incidents of unintended acceleration.

• FLOOR MATS

United States: 5.35 million

Canada: 400,000

Total vehicles recalled: 5.75 million

• STICKING PEDALS

United States and Canada: 2.58 million

Europe: 1.71 million

China, Middle East, Africa, Latin America: 160,000

Total vehicles recalled: 4.45 million

Affected by both recalls: about 2.1 million

Grand total of vehicles recalled: 8.1 million

Source: Toyota Motor Corp., NHTSA

Dropping the ball

"NHTSA dropped the ball," says Clarence Ditlow, the center's executive director. "They had no data or procedures for the test to back up the conclusion that it wasn't electronic controls."

The mounting skepticism spells more trouble for the world's largest automaker as it struggles to stamp out complaints about unintended acceleration linked to scores of accidents, injuries and deaths in Toyota and Lexus vehicles.

Since last fall, the carmaker has recalled 8.1 million vehicles to address unintended acceleration -- more than it sold globally last year. Meanwhile, it has embarked on an unprecedented cessation of sales and production of some of the affected vehicles in the United States.

That same day, the Transportation Department said it would revisit whether faulty electronic controls are linked to unwanted acceleration in Toyotas as well as other brands. NHTSA said the agency will meet with manufacturers, suppliers and independent experts to get a better understanding of vehicles' electronic throttle controls -- which are computerized gas-pedal systems.

Suspicion of Toyota's electronic control system stretches back to the first investigations of unintended acceleration in the early 2000s. Critics argue that Toyota's proposed fixes -- one addressing floor-mat interference and the other sticking pedals -- fail to cover all the cases.

Toyota has embarked on two massive recalls to fix the acceleration problem.

The first recall, announced last fall and expanded in January, targets floor mats in the United States and Canada that could jam the gas pedal into an open-throttle position.

The second recall, announced last month, targets vehicles on five continents and aims to fix sticky accelerators.

Toyota is adamant it knows what's wrong.

"We've studied the events of unintended acceleration, and we're quite clear that it's come down to two different issues," Jim Lentz, president of Toyota Motor Sales U.S.A. Inc., told Matt Lauer last week on NBC's "Today Show." "Between those two things, this will be under control."

In the interview, Lentz ruled out any glitch in the electronic control systems.

Yet questions persist. By Toyota's own admission, it has not tracked the pedal problem to any accidents. And there are other acceleration crashes where the floor mats can be ruled out.

The day after Christmas, for instance, a 2008 Toyota Avalon sped off the road and landed upside down in a Texas pond, killing four people. The cause of that crash is still undetermined. But police have ruled out floor mats, which were found in the trunk.

During the 2007 investigation of unintended acceleration in the Lexus ES 350, NHTSA surveyed 1,986 owners of the car. Of the 600 responses received, 59 said they had experienced unwanted acceleration. But only 35 of those said they were using the all-weather floor mats that eventually were blamed for the surging cars and recalled.

Toyota's system has dual sensors backstopping each other in monitoring the accelerator pedal's position, along with two more sensors double-checking the throttle position.

Meanwhile, a control computer actuates the throttle, and a monitoring computer surveys all the computer signals in the circuit. If any abnormal signals are detected, the engine is immediately returned to idle, Yokoyama said.

Furthermore, Toyota says the throttle control is reliable under extreme conditions of electromagnetic waves, temperature and vibration.

NHTSA probes

Indeed, NHTSA says it has repeatedly tested the electronic control system and says it can't link the technology with runaway cars.

In announcing that it would revisit the problem, NHTSA said it was aware of allegations that electromagnetic interference can have fleeting, hard-to-trace, detrimental effects on a car's electronic throttle control and lead to unintended acceleration.

But it added: "The agency has no reason at this point to believe there are safety defects in these systems or in their ability to function."

Since 2004, NHTSA has conducted at least four investigations into complaints about unintended acceleration and possible electronic throttle problems in Toyota or Lexus vehicles.

NHTSA closed two investigations without finding any defect trends.

In the case where magnets added 1,000 rpm to the engine of a Lexus ES 350, NHTSA found that all-weather floor mats could jam the gas pedal. It didn't blame electronics.

Finally, in the fourth case, unintended acceleration was traced to a floor carpet cover that, if not replaced properly after repair, also could jam the gas pedal.

But Toyota's electronic control systems haven't been completely trouble-free.

In 2002 and again in 2003, Toyota issued technical service bulletins to update the electronic control modules in certain 2002-03 Camrys to prevent the engine from surging, says Sean Kane, president of Safety Research & Strategies.

To appraise NHTSA's efforts, Ditlow's Center for Auto Safety requested the agency's records on Dec. 1, 2009, under the Freedom of Information Act.

Ditlow requested information on NHTSA's test procedures. He also asked for records of meetings and communications between the agency and Toyota.

In NHTSA's reply, dated Jan. 21 and seen by Automotive News, the agency said it found no records describing test protocols. It also produced no additional communications with Toyota beyond the formal filings already listed on the home page of the agency's Web site.

Continuing pressure

"NHTSA can't say what it did, how it did it or what the results were," Ditlow said. "There has been no documented engineering analysis done of whether intermittent failures in the electronic control system cause the unintended-acceleration events."

Nicole Nason, NHTSA administrator from 2006 to 2007, has defended the agency's handling of the 2007 Lexus ES 350 tests and the conclusion that the unintended acceleration was a problem with the floor mat, not the electronics.

"We had to get information out to vehicle owners as quickly as possible," Nason told Automotive News last week. "This was not a broken door-lock or a flickering headlight. This was a very significant safety problem."